The adhesive bonding of composite structures, in particular large or complex structures, is predominantly performed using one of three approaches: 1) secondary bonding; 2) co-bonding; and 3) co-curing. Secondary bonding (shown in FIG. 1A) is the joining together of pre-cured composite parts by adhesive bonding, wherein only the adhesive is being cured. This secondary bonding method typically requires surface preparation of each previously cured composite part at the bonding surfaces to form a strong link. Co-bonding (shown in FIG. 1B) involves joining a pre-cured composite part to an uncured composite part by adhesive bonding, wherein the adhesive and the uncured composite part are being cured during the bonding process. The pre-cured composite usually requires an additional surface preparation step prior to adhesive bonding. The last approach, co-curing (shown in FIG. 1C) joins uncured composite parts by simultaneously curing and bonding, wherein the composite parts are being cured together with the adhesive, resulting in chemical bonding. However, it is difficult to apply this technique to the bonding of uncured prepregs to fabricate large structural parts with complex shapes. Uncured composite materials, e.g. prepregs, are tacky (i.e. sticky to the touch) and lack the rigidity necessary to be self-supporting. As such, uncured composite materials in the co-cure method are frequently difficult to handle. For example, it is difficult to assemble and bond uncured composite materials on tools with complex three-dimensional shapes.
In the aerospace industry, airframe manufacturers making large-scale composite structures commonly apply secondary bonding techniques to join the molded and cured thermoset components. Secondary bonding, while highly effective in most cases, sometimes results in a weak bond at the adhesive/adherent interface. Due to the unpredictable nature of the interface, the Federal Aviation Administration (FAA) certification of primary structures with secondary bonds requires that aircraft manufacturers incorporate redundant load paths accomplished mostly by adding mechanical fasteners. Airframe manufacturers using the secondary bonding approach must additionally install mechanical fasteners on adhesively bonded joints to comply with federal aviation regulations. An average commercial aircraft may contain up to several miles of adhesively bonded joints and thousands of redundant fasteners. An alternate means of assembling large-scale composite structures to meet federal regulations is needed to realize the ultimate potential of composites to reduce cost and increase aircraft performance. In some applications, by removing redundant fasteners in an aircraft, one may reduce the part count by up to 120,000 parts and the weight of the aircraft by up to 5000 lbs. (2%).
The use of secondary bonding and co-bonding techniques frequently result in unpredictable joint strengths that require the addition of mechanical fasteners while the use of the co-cure process is limited by the complexity of the part or the size of the autoclave or oven being used to mold and cure the part. A need exists for alternative manufacturing methods to fabricate unitized composite structures with reliable, certifiable joints without the need for redundant mechanical fasteners.